# std::chrono::year_month_day::operator+=, std::chrono::year_month_day::operator-=

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< cpp‎ | chrono‎ | year month day

constexpr std::chrono::year_month_day& operator+=( const std::chrono::years& dy ) const noexcept; | (1) | (since C++20)
constexpr std::chrono::year_month_day& operator+=( const std::chrono::months& dm ) const noexcept; | (2) | (since C++20)
constexpr std::chrono::year_month_day& operator-=( const std::chrono::years& dy ) const noexcept; | (3) | (since C++20)
constexpr std::chrono::year_month_day& operator-=( const std::chrono::months& dm ) const noexcept; | (4) | (since C++20)

Modifies the time point *this represents by the duration dy or dm.

1) Equivalent to *this = *this + dy;.
2) Equivalent to *this = *this + dm;.
3) Equivalent to *this = *this - dy;.
4) Equivalent to *this = *this - dm;.

For durations that are convertible to both std::chrono::years and std::chrono::months, the `years` overloads (1,3) are preferred if the call would otherwise be ambiguous.

### Example

Run this code

    #include <cassert>
    #include <chrono>
    #include <iostream>

    int main()
    {
        constexpr auto monthsInYear{12};
        auto ymd{std::chrono::day(1)/std::chrono::July/2020};
        std::cout << "#1 " << ymd << '\n';

        ymd -= std::chrono::years(10);
        std::cout << "#2 " << ymd << '\n';
        assert(ymd.month() == std::chrono::July);
        assert(ymd.year() == std::chrono::year(2010));

        ymd += std::chrono::months(10 * monthsInYear + 11);
        std::cout << "#3 " << ymd << '\n';
        assert(ymd.month() == std::chrono::month(6));
        assert(ymd.year() == std::chrono::year(2021));

        // Handling the ymd += months "overflow" case.
        ymd = std::chrono::May/31/2021; // ok
        std::cout << "#4 " << ymd << '\n';
        assert(ymd.ok());

        ymd += std::chrono::months{1}; // bad date: June has only 30 days
        std::cout << "#5 " << ymd << '\n';
        assert(not ymd.ok());
        assert(ymd == std::chrono::June/31/2021);

        // Snap to the last day of the month, June 30:
        const auto ymd1 = ymd.year()/ymd.month()/std::chrono::last;
        std::cout << "#6 " << ymd1 << '\n';
        assert(ymd1.ok());
        assert(ymd1 == std::chrono::June/30/2021);

        // Overflow into the next month, July 1 (via converting to/from sys_days):
        const std::chrono::year_month_day ymd2 = std::chrono::sys_days{ymd};
        std::cout << "#7 " << ymd2 << '\n';
        assert(ymd2.ok());
        assert(ymd2 == std::chrono::July/1/2021);
    }

Output:

    #1 2020-07-01
    #2 2010-07-01
    #3 2021-06-01
    #4 2021-05-31
    #5 2021-06-31 is not a valid date
    #6 2021/Jun/last
    #7 2021-07-01

### See also

operator+operator- (C++20) | adds or subtracts a `year_month_day` and some number of years or months (function)

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